This invention relates in general to a heat exchanger, and in particular, to an annular tube heat exchanger for a gas-fired steam generator.
Typically, heat exchangers with gas carrying tubes for heating a fluid, for example, water to produce steam may include a plurality of tubes or baffles for directing the flow of combustion gas through the heat exchanger. The tubes or baffles may be arranged in a variety of different configurations. For example, in U.S. Pat. No. 5,816,496 to Kovacs, a flue gas apparatus generally includes planar, vertical inner side walls that can be constructed in an inverted U-shaped configuration. The steam generated in the steam generator is supplied to a humidification apparatus or distributor where the steam is discharged into a moving air stream. The condensate from the steam distributor can be directed back into the water tank of the steam generator. In Kovacs, the combustion burner is supplied with an air-gas mixture regulated by a constant ratio valve and a forced draft means, such as a fan. Further, the combustion process is controlled with a modulating type humidistat associated with the steam distributor to dampen out spikes in demand for more steam. A signal from the humidistat detecting a condition of low humidity generates an increase in the air/gas flow into the burner to generate more steam.
However, it is desirable in any type of heat exchanger to provide the most energy efficient arrangement by maximizing the ratio of heat transfer surface area to the space occupied by the heat exchanger. In addition, it is desirable to provide a heat exchanger in which the combustion gases provide an efficient high velocity heat transfer to the fluid medium. Further, it is desirable to provide a heat exchanger with a simple mechanical structure to reduce manufacturing costs.